I have been connecting to the source in http://rapiddevbookcode.codeplex.com from inside Visual Studio reasonably well for years then about a month ago I started getting:

TF31003: Either you have not entered the necessary credentials or your user account does not have permission to connect to the Team Foundation Server at https://tfs.codeplex.com/. Click the Use different credentials link below, or ask your server administrator to add the appropriate permissions to your account

on one machine only, Team Explorer on 3 or 4 other machines still connects fine.

On the machine that doesn't work TFS Power Tool Windows Shell Extension does, so I've been using that.
I was also able to connect from SSMS using Microsoft Visual Studio Team Foundation Server 11 Beta MSSCCI Provider 32-bit http://visualstudiogallery.msdn.microsoft.com/ca270d2b-b74b-4307-a61c-9204f1255c15

Machine is Windows 7 64 bit with VS 2010 SP1 and VS 11 - I get error with both version of VS.
I've had that message before but it usually was fixed by restarting Visual Studio.

I also got this error in VS 11 at one point

TF205020: A connection could not be made to the following server: tfs.codeplex.com\TFS01. This server was used during your most recent session, but it might be offline, or network problems might be preventing the connection. Contact the administrator for Team Foundation Server to confirm that the server is available on the network. To attempt to connect to this server or a different server, open the Team menu, and click Connect to Team Foundation Server.

The server returned the following error: TF30063: You are not authorized to access tfs.codeplex.com\TFS01.